Jeongkyo Seo

Improvement of Cover-Layer Surface Properties for Near-Field Optical Recording

Scratches in a soft polymer cover-layer on near-field recording (NFR) media are observed after a servo test with a rotating disc owing to collision between a solid immersion lens (SIL) and a NFR disk. A dielectric protective film is coated on the cover-layer to enhance surface properties, which reduces the scratch problem of the cover-layer. The refractive index of the dielectric film can be tailored by controlling process parameters during the sputtering and matches that of the polymer cover-layer to reduce light loss at an interface between the dielectric film and the polymer cover-layer.

Near-field optical recording with nanocomposite cover-layer for numerical aperture of 1.85

Near-field optical recording media with a nanocomposite cover-layer were prepared and tested using a gap servo, near-field recording system with an effective numerical aperture of 1.85. The refractive index of the nanocomposite cover-layer, which has a smooth surface, is 1.86, yielding a successful gap servo operation. However, the recording density cannot be maximized due to the refractive index of the cover-layer being lower than the required value for the effective numerical aperture. A recording density of 95 GB per disc can be achieved with a nanocomposite cover-layer having a refractive index of 1.86. The readout signal from 110 GB is not clear due to the reduced eventual numerical aperture in conjunction with the limited refractive index of the cover-layer.

Numerical Simulations and Experiments about Contamination Mechanism of Solid Immersion Lens System Concerning Hole and Geometries of Lens Holder

The air flow field around a conical type solid immersion lens (SIL) system is simulated numerically and confirmed with experiment using a micro-particle image velocimetry (PIV) system. It is found that the back-flow from the downstream of the SIL is a major candidate for the contamination of the SIL. Five modifications are proposed to suppress the particle conveying mechanism, the back-flow. Among these modifications a method using two flow-bypasses reduces the back-flow most effectively, which connects the top surface of the SIL system and two side-holes of the lens holder where the static pressure is the minimum.

Dynamic Tilt Control of SIL with 4-axis Actuator in NFR System

In the near field recording (NFR) system, tilt margin between disc and solid immersion lens (SIL) is affected by the geometric features of the SIL. If the tilt angle is larger than the allowable tolerance, SIL can touch the surface of a disc and stable read-out of the recorded data or data writing may be difficult. Because tangential as well as radial tilt angle of real disc is so considerable in NFR system with low tilt tolerance, we propose a 4-axis actuator for active tilt compensation to diminish radial and tangential tilt errors as well as focusing and tracking errors between disc and SIL in this paper. Disc tilt and tilt detectable gap error signal (GES) are measured simultaneously and compared to examine the relation of two signals. For this, we newly designed and tested a 4-axis actuator and also established GES-based tilt error detecting test-bed. As a result, we confirmed the mechanical characteristics of the 4-axis actuator and the decrease of tilt errors in radial and tangential tilt directions. We expect that this tilt error reduction can improve the productivity and reliability of this NFR system and relieve disc manufacturing tolerance towards SOOGB capacity.

High Density Recording with Guided-Layer Media

Recording characteristics of guided-layer media consisting of planar recording layer and land/groove patterned guided-layer were investigated. Guided-media can be operated with a focusing servo and a tracking servo on the planar recording layer and the guided-layer, respectively. Recording properties using not only two blue lasers but also one blue and one red laser were measured. The jitter of about 8% was obtained from the planar recording layer on the guided-layer using two blue lasers system. Even though the jitter was not obtained, a similar performance to the previous result was observed with the blue laser and the red laser system. Though these experimental results, the possibility of high recording density with increasing data transfer rate could be confirmed.

High density recording with SIL-based near-field optical recording

High density rewritable recording and readout characteristics with a solid immersion lens (SIL)-based near-field recording (NFR) are investigated. Substrates for high density are prepared using a phase transition mastering technique. Cover-layers with different refractive index for different effective numerical apertures in the SIL optics are coated on rewritable NFR discs for cover-layer incident near-field recording configuration. Two different effective numerical apertures are chosen between 1.45 and 1.85 to cope with about 70 GB and more than 100 GB per disc, respectively. The performance readout signals is investigated and compared in terms of increasing recording densities with reducing track pitch.

Aberration compensation in near-field optics for multilayer data storage

Spherical aberration (SA) is induced by a change in the focused layer in a multilayer system. In a multilayer far-field system, any kind of aberration-generating system can well compensate the SA induced by a change in the focused layer. However, the method used in the multilayer far-field system cannot be applied to a multilayer near-field recording (NFR) system with a solid immersion lens (SIL), because of many critical reasons including sensitivity. In a multilayer NFR SIL system, another method is required to compensate aberration. The use of a liquid crystal plate (LCP), afocal telescope-type lens compensator or a compact combined lens system is considered for the compensation of SA in the multilayer NFR SIL system in this study.

Improvement of optical resolution and mechanical robustness for near-field recording system

Near-field optics applied annular aperture technique is optimized to improve near-field optical imaging performance. By introducing an annular aperture that has three annular zones with binary transmittance and different phase retardation, decreased focused spot size, reduced sidelobe intensity and extended focal depth are achieved. In addition, through the development of novel near-field air-gap control algorithm, near-field recording system, which inevitably maintains extremely small air-gap, becomes much stable with the enhanced robustness for external shock and vibration.

Media related characteristics of the solid immersion lens system

First, it is concluded that re-designing the objective lens of system is a good method to obtain maximum performance at fixed refractive index of media. Also, two systems cutting SIL and none cutting SIL were compared as re-designing objective lens. The result says that two systems are the same practically. Therefore, in designing the system, it does not need to consider the cutting the SIL. In comparing with Opti and Hyper, Opti has more tolerance that Hyper in certain respects. Hence Opti can be a suitable system for Optical data storage. Adding to this, it is possible to minimize the error due to production process by moving the collimate lens.

The Angle Align Method of Reference Beam for Holographic Data Storage

We introduce a method to align the angle of reference beam holographic data storage by using a photodiode in front of image sensor, the brightness of information data can be easily extracted. From the output of the photodiode, we can quickly find the optimal angle for readout beam